The present invention relates to a surface acoustic wave element having a bump electrode and a surface acoustic wave device in which a SAW element is mounted on a base substrate through a solder bump connection.
A conventional surface acoustic wave (SAW) device includes a SAW element which is mounted on a base substrate via a solder bump connection and the SAW element is sealed by bonding a metal cap to the base substrate via soldering. FIG. 4 is a sectional view showing a bump electrode of a conventional SAW element in such a SAW device.
The conventional SAW element shown in FIG. 4 includes a surface wave substrate 11, an interdigital transducer (IDT) electrode (not illustrated), a reflector electrode (not illustrated) and a lead electrode 14 connected to the IDT electrode and disposed on one main surface of the surface wave substrate 11. A bump electrode 40 is disposed at an end portion of the lead electrode 14. The lead electrode 14 and the bump electrode 40 are formed by a method of vapor deposition, sputtering or the like. The lead electrode 14 is made of Al. The bump electrode 40 normally comprises a plurality of metal layers, such as a NiCr layer 40a, an Ni layer 40b and an Ag layer 40c having the same shape, as shown in FIG. 4.
The SAW element shown in FIG. 4 is fixedly supported by a base electrode and electrically connected thereto via soldering (solder bump connection) the bump electrode 40 of the SAW element to a corresponding electrode land on the base substrate. More specifically, after preliminary soldering is carried out on the electrode land of the base substrate, the bump electrode of the SAW element is soldered to the electrode land of the base substrate via a reflow soldering process.
A metal cap is then soldered to a bonding area of the base substrate to cover the SAW element and the SAW element is therefore sealed inside of a closed inner space formed by the base substrate and the metal cap.
Although such a solder bump connection is a well-established technique and is used widely in various kinds of electronic components, the conventional SAW device has a problem with the solder bump connection. More specifically, when the SAW element is soldered to the base substrate, molten solder flows along a peripheral side of the bump electrode 40 and reaches a lead electrode of the SAW element such that a portion of the solder is in contact with the lead electrode. Since the lead electrode is made of a thin aluminum layer, there arises a problem that the lead electrode reacts with or is eroded by the solder thereby causing a connection failure of the lead electrode 14 and degradation of the device characteristic due to the connection failure.
The preferred embodiments of the present invention overcome the problems described above by providing a surface acoustic wave element which eliminates corrosion of an Al electrode caused by a reaction with solder when the element is mounted on a base substrate via soldering.
The preferred embodiments of the present invention overcome the problems described above by providing a surface acoustic wave device which prevents degradation of the device characteristics caused by a connection failure of a lead electrode.
A preferred embodiment of the present invention provides a surface acoustic wave element including a surface acoustic wave substrate, an interdigital transducer electrode provided on the surface acoustic wave substrate and a lead electrode provided on the surface acoustic wave substrate. The lead electrode is connected to the interdigital transducer electrode. The surface acoustic wave element further includes a bump electrode provided on a portion of the lead electrodes. The bump electrode includes a first electrode structure and a second electrode structure. The first electrode structure has a top surface and is in contact with the lead electrode, and the second electrode structure covers a portion of the top surface of the first electrode structure such that the remaining portion of the top surface of the first electrode structure is exposed along a whole periphery of the top surface of the first electrode structure.
The surface acoustic wave device according to another preferred embodiment preferably includes a surface acoustic wave element constructed according to the preferred embodiment described above, a base substrate having electrode lands and a bond land on a surface thereof and a cap. The surface acoustic wave element is mounted on the base substrate such that the bump electrodes of the surface acoustic wave element are soldered to the respective electrode lands of the base substrate. The cap has an opening and a flange portion defining the opening, and the flange portion is soldered to the bond land of the base substrate such that the surface acoustic wave element is sealed by the cap and the base substrate.
The first electrode structure may preferably include a layer having poor solder wettability and defining a top surface thereof. This layer is preferably made of Al. The first electrode structure may further include a pair of NiCr layers and a Ni layer disposed between the pair of NiCr layers. Alternatively, the first electrode structure may preferably include only a NiCr layer or a Ni layer.
The second electrode structure may preferably include an Ag layer which defines a top surface of the second electrode structure. Moreover, the second electrode structure may further include a NiCr layer and a Ni layer disposed between the Ag layer and the NiCr layer.
The first electrode structure further preferably has a side surface which is substantially perpendicular to the top surface. The top surface of the first electrode structure may have a substantially circular shape and the second electrode structure has a top surface having a substantially circular shape which is smaller than the top surface of the first electrode structure. The top surface of the first electrode structure and the top surface of the second electrode structure are preferably arranged so as to be aligned with each other at center thereof.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.